Mobile communication networks typically comprise a plurality of base stations and a plurality of mobile stations. In order for the mobile stations to communicate effectively with the base stations, they must be tuned precisely to the frequencies used by the base stations. In the case of GSM, the tolerance for traffic mode operation is 0.1 ppm.
Although crystal-controlled reference oscillators are used in the mobile stations, tuning errors can be significant. Indeed, when a mobile station is in sleep mode, the local oscillator drift could be up to 500 Hz in the 900 MHz band, chiefly due to temperature effects and mis-setting. Additional, frequency errors can arise due to drift of the base station's oscillators and Doppler shift.
Consequently, the base stations transmit frequency correction signals.
In GSM (Global System for Mobile Communications) and similar systems, the frequency correction signal comprises a frequency control channel (FCCH) which the mobile station uses when it is powered up.
When a mobile station initiates a connection, such as “Push to Talk” or WAP, the mobile station must already be on. For such a mobile station initiated connection, the mobile station will be configured in ALL_(P)CCCHmode.
The problem of synchronising with the base station arises when the uplink connection is initiated by the mobile station and the mobile station is configured to ALL_(P)CCCH. The worst case is that the mobile station is configured to combined BCCH/CCCH/SDCCH/SACCH, where 4 block periods (one block is 4 bursts) are used by BCCH and CCCH and 6 block periods are reserved for SDCCH and SACCH in the 51-multiframe. This means only 4 blocks (16 bursts) from the BCCH and the CCCH are available for AFC (automatic frequency correction). Therefore, it takes long time for AFC to adjust the frequency offset to less than 0.1 ppm. Furthermore, the conventional AFC uses only the bursts available in TS0 (time slot 0).